1. Field of the Invention
This invention generally relates to apparatus and methods for encoding or embedding information in cover signals, such as audio, video and data signals, either transmitted by radio wave transmission or wired transmission, or stored in a recording medium such as optical or magnetic disks, magnetic tape, or solid state memory. In particular, the present invention relates to methods and apparatus for preventing the embedded information from being removed or obliterated from the cover signal.
2. Background and Description of Related Art
With the advent of the Internet and improvements in digital technology, it is becoming possible to make unlimited copies of copyrighted content such as audio, video, still images and the like, without degradation of the original content. For example, there is currently a large market for audio software products, such as musical recordings. One of the problems in this market is the ease of copying such products without paying those who produce them. This problem is becoming particularly troublesome with the advent of recording techniques, such as digital audio tape (DAT), which make it possible for copies to be of very high quality. It is thus desirable to be able to prevent the unauthorized copying of audio recordings, including the unauthorized copying of audio works broadcast over the airwaves or made available on the Internet.
Recently, a significant amount of attention has been given to various methods for digital "watermarking" of copyright protected program material. As opposed to encryption, in which the content of the program itself is subjected to scrambling or encoding, such that without the correct decoder equipment, the content cannot be reproduced, watermarking relates to the insertion into program material such as audio or video signals digital information identifying the copyright holder and/or the purchaser of a particular copy of the audio or video program in a manner which is imperceptible to the listener or observer. The watermarking information is thus hidden from the ordinary recipient of the program, and can be detected only by use of appropriate apparatus or knowledge of the watermarking parameters. Such watermarking is also known as steganography. Literally meaning "covered writing" from the Greek, steganography has been defined as the art and science of communicating in a way which hides the existence of the communication.
Various prior art methods of encoding auxiliary hidden information onto a source or cover signal are known in the art. See, e.g., U.S. Pat. Nos. 5,319,735; 5,450,490; 5,613,004; 5,687,236; 5,636,292; and 5,574,962.
In the area of copyright enforcement, the watermark information may contain the identity of the purchaser of a particular copy of the audio, video, or other program material. Thus, each copy of the same audio or video program would contain a different watermark to the extent that such copy is purchased by a different entity.
The value of any watermarking technique lies in its resistance to removal from the cover signal, without also destroying the content of the audio or video program in which it resides. In this regard, there exists a problem in the art known as a collusion attack. A collusion attack results when two or more copies of a program signal with different watermarks are combined in some manner, such as averaging or interleaving, whereby the watermark information is obliterated, while the quality of the content information is preserved. Because the program content is identical, averaging the signal will produce the same content. However, the watermarking information would be destroyed because this information would be different for each copy of the cover signal, so that averaging of the copies would in effect "erase" the watermark.
Prior techniques for defending against such collusion attacks are known in the art. Such techniques have focused on adding redundancy to watermarking codes to obtain similarity in at least a portion of the watermark information in different copies. For example, U.S. Pat. No. 5,664,018 to Leighton discloses a method wherein a secret "baseline" watermark is created from a digital string of the work to be marked, which is then used to create a set of modified watermarks, each having a certain relationship to the baseline watermark. Each modified watermark is then inserted into a copy of the work to create a watermarked copy of the work. If any small subset of the modified watermarks are averaged, at least one of the watermarks will remain.
Several significant disadvantages exist in the known methods for preventing watermark removal. First, use of data redundancy imposes a large penalty on the required length of the watermark data with respect to the actual embedded information. Second, such anti-collusion methods actually cannot prevent removal of watermarks if a sufficiently large number of copies is used in the averaging. Collusion attacks using a sufficient number of different copies will completely remove the watermarks without causing any degradation of the cover signal content. As such, there exists a need in the art for improvement in methods and corresponding apparatus for preventing the removal of watermark information from cover signals.